It's well-known that LaTeX commands tend to be more complex, and to
run more slowly than, any Plain (or primitive) command that they
replace. There is therefore great temptation not to use LaTeX
commands when macro programming. Nevertheless, the general rule is
that you should use LaTeX commands, if there are seeming
equivalents. The exception is when you are sure you know the
differences between the two commands and you know that you need the
Plain version. So, for example, use \mbox in place of \hbox
unless you know that the extras that LaTeX provides in \mbox
would cause trouble in your application. Similarly, use
\newcommand (or one of its relatives) unless you need one of the
constructs that cannot be achieved without the use of \def (or friends).
As a general rule, any LaTeX text command will start a new paragraph if necessary; this isn't the case with Plain TeX commands, a fact which has a potential to confuse.
The commands \smallskip, \medskip and \bigskip exist both
in Plain TeX and LaTeX, but behave slightly differently: in Plain
TeX they terminate the current paragraph, but in LaTeX they
don't. The command \line is part of picture mode in LaTeX,
whereas it's defined as "\hbox to \hsize" in
Plain TeX. (There's no equivalent for users of the Plain TeX command in
LaTeX: an equivalent appears as the internal command \@@line).
Maths setting shows a case where the LaTeX version is
essentially equivalent to the TeX primitive commands: the LaTeX
\( ... \) does essentially no different to the
TeX $ ... $
(except for checking that you're not attempting to open a maths
environment when you're already in one, or vice versa).
However, \[ ... \] isn't the same as
$$ ... $$: the TeX version, used
in LaTeX, contrives to miss the effect of the class option
fleqn.
Font handling is, of course, wildly different in Plain TeX and
LaTeX: even the LaTeX equivalent of the Plain TeX
font-loading command (\newfont) should be avoided wherever
possible: the possibilities of confusion with constructs that vary
their behaviour according to the font size that LaTeX has recorded
are (sadly) legion. A really rather funny example is to be had by
saying:
\documentclass{article}
\begin{document}
\font \myfont=cmr17 scaled 2000
\myfont
\LaTeX
\end{document}
(the reader is encouraged to try this). The "A" of LaTeX
has pretty much disappeared: LaTeX sets the "A" according to
its idea of the font size (10pt), but "\myfont" is more
than three times that size.
Another "\myfont" example arises from an entirely different
source. The mini-document:
\documentclass{article}
\begin{document}
\font \myfont=ecrm1000
{\myfont par\`a}
\end{document}
gives you "German low double quotes" in place of the grave accent.
This problem arises because ecrm1000 is in a different
font encoding than LaTeX is expecting - if you
use LaTeX font commands, all the tiresome encoding issues are
solved for you, behind the scenes.
So, whether you're dealing with a one-off font or a major new family, you are far more likely to be satisfied with the LaTeX file selection system, so it's worth investing a small amount of effort to write declarations of the font's family and how it should be loaded. For details of the commands you need, see the LaTeX font usage guide, fntguide: this may be viewed on the archive, but you should find a copy of the document in your own system.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=plninltx